The present invention relates to an improvement of a corrective tempering method and apparatus for correcting the hardening deformation of rolling elements.
In the process of manufacturing rolling elements such as the inner and outer races, cages, and other parts of, e.g., rolling bearings, the deformation of the rolling elements induced by such a heat treatment as hardening has heretofore been corrected by tempering. Conventional tempering methods include so-called "bracing tempering" and press tempering. The former method is characterized as correcting the deformation of a rolling element by subjecting the rolling element to such a normally tempering treatment as keeping the rolling element in the tempering furnace at a required temperature for 60 to 120 minutes with the rolling element being given a reversely bracing deformation using an appropriate tool. On the other hand, the press tempering method is characterized as correcting the deformation of a rolling element by subjecting the rolling element to the aforementioned normally tempering treatment with the rolling element pressured by molds or the like before the tempering treatment.
However, the conventional tempering methods address several problems. One problem is that the treatment time required for correcting the deformation of a rolling element is very long, not only because these methods involve the step of preparing and setting tools, molds, and the like in consideration of the size and amount of deformation of the rolling element to be corrected and pressuring the rolling element with these tools and molds, but also because the rolling element is thereafter subjected to the normally tempering treatment for 60 to 120 minutes in the furnace. More specifically, the following problems have been encountered.
(1) A number of tools and molds must be involved to treat a number of rolling elements on a mass production basis. Therefore, the conventional arts that are of low productivity cannot respond to the needs for mass production. PA1 (2) Further, the tempering furnace, being heated with hot blast, takes 20 minutes or more for heating a rolling element, and there is a limit in shortening the treatment time imposed by this slow heating of the rolling element. PA1 (3) Still further, if atmosphere temperature is increased to shorten the treatment time, the hardness of the rolling element is reduced. PA1 (4) Still further, the heat-treated rolling element still holds inconsistencies caused in turning before the heat treatment, which in turn requires that grinding margins after the heat treatment be increased. As a result, grinding cost reduction is restricted.